Variation in larval traits between closely related species of freshwater prawns (Macrobrachium amazonicum and M. pantanalense)

Larval morphology is a valuable tool for understanding the life history of decapod crustaceans. This approach has proven valuable in confirming taxonomic revisions based on molecular or adult morphology analyses. Therefore, the present study aimed to compare larval traits (e.g. development time, morphology, and larval size) between Macrobrachium amazonicum and M. pantanalense, two closely related species that are separated by a low genetic distance. The first five zoeal stages of each phenotype and species were analysed. Differences in larval development time, morphology, and size were observed. M. amazonicum exhibits faster development during the early larval stages compared to M. pantanalense. The main morphological difference between the two species is related to the stage at which pereiopod five develops as a functional appendage, namely zoea IV in M. amazonicum and zoea III in M. pantanalense. In addition, size variation was observed, with M. pantanalense larvae being larger in the early stages. The differences found between the two species corroborate that M. amazonicum and M. pantanalense are distinct lineages. Even though these groups are separated by a low genetic distance, the existing differences are conclusive, and therefore, these organisms can be considered as two distinct taxonomic entities.